To control an internal combustion engine, it is known from German Published Patent Application No. 42 39 711 (U.S. Pat. No. 5,558,178) to convert a setpoint value for a torque of the internal combustion engine into an actuating variable for influencing the air supply to the combustion engine, to set the ignition angle and/or to suppress or connect the fuel supply to individual cylinders of the internal combustion engine. In addition, the controlling of the fuel/air ratio to realize the predefined torque value is also known from PCT Publication No. 95/24550 (U.S. Pat. No. 5,692,471). Furthermore, in the known approaches the actual torque of the internal combustion engine is calculated taking the actual engine settings into account (charge, fuel metering and ignition angle). Among others, the engine speed, load (air mass, pressure etc.) and possibly the exhaust-gas composition are utilized.
In the context of these calculations, a torque model for the internal combustion engine is used, which is utilized both for determining the actuating variables and also for ascertaining the actual values. The essence of this model is that values for an optimum torque of the internal combustion engine and for an optimum ignition angle are determined as a function of operating points, which are then corrected via efficiency values in accordance with the actual setting of the internal combustion engine.
To optimize this model, it is known from German Published Patent Application No. 195 45 221 (U.S. Pat. No. 5,832,897) to correct the value for the optimum ignition angle as a function of variables influencing the efficiency of the internal combustion engine such as exhaust-gas recirculation rate, engine temperature, intake-air temperature, valve-overlap angle, etc.
In practice, however, it has been discovered that this known approach may yet be optimized further, especially with respect to simplifying the application, optimizing the computing time and/or considering the working-point dependency of the correction of the optimum ignition angle, especially the dependency on the inert-gas rate. In some operating states, in particular, the known torque model does not deliver satisfactory results. Such operating states are especially states with high inert-gas rates, i.e., states having a large proportion of inert gas (by external or internal exhaust-gas recirculation), which are caused by an overlap of intake and discharge-valve opening times and, above all, with small to medium fresh-air charges. Furthermore, these are operating states having high charge movements. Since these effects are not fully taken into account, the calculated basic variables make it impossible for the known procedure to obtain a precise torque calculation.
Another problem is that the known model, while taking the essential influences on the ignition angle into account when determining the maximum torque, does not consider the different slopes of the correlation between ignition angle and the maximum engine torque which is able to be attained with different mixtures in the instantaneous operating point of the engine. In optimizing the known model, a compromise must thus be found, which usually consists in the optimum ignition angle no longer coinciding with the ignition angle of the optimum torque. For example, in mixtures having good combustibility, which have a highly retarded ignition angle when generating the best torque, the optimum ignition angle is markedly more retarded than this ignition angle, whereas, in mixtures having poor ignitibility, it is considerably more advanced than the ignition angle at which the maximum torque is generated. This characteristic is sketched in FIG. 3. The dashed line there represents the curve provided by optimizing the known model. As can be seen, the actual and the model curve no longer correspond precisely. In one example of application, a resulting torque fault of up to 20% relative to the optimum torque MIopt has come about at the best possible optimum ignition angle.
Thus, it is obvious that the greater the variation in the steepness of the ignition hook (correlation between optimum ignition angle ZWopt and torque MI with respect to top dead center of ignition phase ignition-TDC for an operating point of the engine), the greater the deviation between the optimum ignition angle and the maximum ignition angle at which maximum torque is attained in mixtures that combust extremely well and those that combust poorly, and the greater the errors of the torque model. The known optimizer of the model parameters is unable to find a population of the model, in particular the optimum ignition angle, so that the torque model has low tolerance across the entire ignition-angle range.
As a result, the torque model must be optimized further, in particular with respect to engine-control systems having high inert-gas rates, such as engine-control systems having variable valve timing and/or charge-movement flap.
From the not pre-published German Patent Application No. 101 49 477.7, a method and a device as well as a computer program for controlling an internal combustion engine are known, a torque model being used within the framework of calculating instantaneous variables and/or actuating variables. In doing so, a basic value ascertained under standard conditions is corrected as a function of the inert-gas rate and/or the valve-overlap angle. Moreover, to further improve the precision of the model, the efficiency of the conversion of the chemical into mechanical energy by which the optimum torque value is corrected is determined as a function of the deviation between an optimum ignition angle and an instantaneous ignition angle as well as an additional variable that represents the combustion performance of the mixture, the latter being the optimum ignition angle in this case.